In recent years, there has been a rapid advancement in the development of portable and cordless consumer electronic devices. For power sources for driving these electronic devices, smaller and lighter batteries having a high energy density have been increasingly demanded. Among these, great expectations are placed on the growth of lithium ion secondary batteries as a power source for portable electronic equipment such as laptop computers, cellular phones and AV equipment because the lithium ion secondary batteries have a high voltage and a high energy density.
For a positive electrode active material of lithium ion second batteries, a lithium-containing composite oxide such as LiCoO2, LiNiO2, LiMnO2, LiMn2O4 has been used. In such a positive electrode active material, destruction of crystal structure or cracking of particle occurs in association with expansion and contraction due to charge and discharge. Repeated charge and discharge cycles therefore result in reduction in capacity and increase in internal resistance.
In order to improve cycle characteristics and safety of batteries, for example, one proposal suggests replacing a part of Co or Ni included in a lithium-containing composite oxide with an element such as Mg to stabilize the crystal structure of the lithium-containing composite oxide (See Patent Document 1).
Among the positive electrode active materials as described above, LiNiO2 has a large theoretical capacity; however, the reversibility of a change in crystal structure associated with charge and discharge is significantly decreased. In order to solve such a problem, another proposal suggests replacing a part of Ni with an element such as Co to mitigate the change in crystal structure (For example, see Patent Document 2).
Moreover, yet another proposal suggests replacing nickel and/or cobalt of a lithium-containing nickel cobalt oxide with inexpensive Mn to obtain Li(NiMnCo)O2 and using this oxide as a positive electrode active material (For example, see Patent Document 3). As a result, a battery being inexpensive and excellent in performance can be obtained.
In many cases, for a separator for use in lithium ion secondary batteries, a porous film made of a thermoplastic resin such as polyolefin is used in view of the safety. This is because that such a separator has a so-called shut-down function. Herein, the shut-down function refers to a function in which when, for example, an external short circuit occurs and the battery temperature is abruptly increased in association with the occurrence of short circuit, the separator is softened and its micropores are closed, causing reduction in ion conductivity to stop the current from flowing.
However, even when the shut-down function is activated, if the battery temperature is further increased, a so-called melt-down occurs, in which the separator is molten and shrank by heat, causing a massive short circuit between the positive electrode and the negative electrode. On the other hand, another problem arises if the heat meltability of the separator is increased in order to improve the shut-down function. That is, the meltdown temperature of the separator is lowered.
In view of the above, for the purpose of improving both the shut-down performance and the meltdown resistance, there have been many proposals for a composite separator including a porous layer made of polyolefin and a layer made of a heat resistant resin. For example, there is a proposal for a separator obtained by laminating a layer composed of a heat resistant nitrogen-containing aromatic polymer such as aramid or polyamideimide and ceramic powder and a layer of porous film (For example, see Patent Document 4).    Patent Document 1: Japanese Patent Laid-Open Publication No. 2002-198051    Patent Document 2: Japanese Patent Publication No. 3,232,943    Patent Document 3: Japanese Patent Laid-Open Publication No. 2004-31091    Patent Document 4: Japanese Patent Publication No. 3,175,730